On Theories of the Origin of Microseisms 



123 



Bernard, P., 1941, Etude sur V Agitation Microseismique 



(These). 

 Donn, W., 1951, J own. Meteor. 8, 406. 

 Gherzi, E., 1921, Etudes stir les microseismes, Note de 



Sism. 06s. Wei-ka-wei. 

 Gutenberg, B., 1952, Observations and theory of micro- 

 seisms, Cat. hist. Techn. 

 LAMB, H., 1904, Phil. Trans. London A, 203, 1. 

 Longuet-Higgins, M. S. & Ursell, F. 1948. Nature, 



162, 700. 

 Longuet-Higgins, M. S., 1950, Phil. Trans. London A, 



2i3, 1. 

 Miche, M., 1944, Ann. Ponts et Chauss, 2, 42. 

 Press, F. and Ewing, M., 1948, Trans. Am. Geoph. Un. 



29, 163. 

 SCHOLTE, J. C, 1943, Versl. Akad. Amsterdam 52, 669. 

 Stoneley, R., 1926, M.N.R.A.S. Geoph. Suppl. 1, 349. 

 Whipple, F. J. W. and Lee, A. W ., 1935, M.N.R.A.S. 



Geoph. Suppl. 3, 287. 



(c) The Mode of Transmission Over Conti- 

 nental Paths — It has been observed that short 

 period surface waves (with periods in the mi- 

 croseism range) propagate with surprisingly 

 little attenuation over large continental paths. 

 (Press and Ewing 1952). Although no 

 theory has been presented to account for the 

 details, it is apparent that the continental crust 

 behaves as an homogeneous sialic plate for 

 these waves. It seems possible that micro- 

 seisms, once past the barrier at the continental 

 margin, may well be transmitted in a manner 

 similar to these earthquake phases. 



REFERENCES 



Discussion 



Frank Press 

 Columbia University 



Dr. Scholte is to be complimented for his 

 concise discussion of Theories of the Origin of 

 Microseisms. We can only add a few remarks 

 on certain aspects of the problem. It is con- 

 venient to discuss separately (a) the nature 

 of the source (b) the mode of transmission 

 over oceanic paths (c) the mode of transmis- 

 sion over continental paths. 



(a) The Nature of the Source — The wave in- 

 terference theory of Longuet-Higgins (see his 

 paper in this volume) is the only published 

 treatment which can quantitatively account 

 for energy transfer from the atmosphere to 

 the ocean. Little work has been done on the 

 possibility that pressure fluctuation and gusti- 

 ness present in turbulent air masses can trans- 

 fer energy directly to compressional waves in 

 the ocean. In this connection the impulsive 

 modification of sea waves by wind gusts 

 (WHIPPLE AND LEE, 1935) may be impor- 

 tant. 



(b) The Mode of Transmission Over Oceanic 

 Paths — There seems to be general agreement 

 among investigators on the manner of trans- 

 mission of elastic energy across the oceans 

 (Stoneley 1926, Press and Ewing 1948, 

 1950a, b, Scholte 1943, Longuet-Higgins 

 1950). That transmission peaks oc- 

 cur for certain periods has been pointed out on 

 several occasions. In order for these trans- 

 mission peaks to develop fully, propagation 

 paths of the order of 100 wavelengths or more 

 are needed, a condition not fulfilled by many 

 microseismic situations. It seems therefore 

 that these transmission peaks can play only a 

 secondary role in any theory on the origin if 

 microseisms. It is particularly disturbing that 

 surface waves from oceanic earthquakes con- 

 tain comparatively little energy in the micro- 

 seism period range. 



Longuet-Higgins, M. S., A Theory of the Origin of 

 Microseisms, Phil. Trans. Roy. Soc. Ser. A., 243, 

 pp. 1-35, 1950. 



Press, F., and Ewing, M., A Theory of Microseisms, 

 with Geologic Applications, Trans. Amer. Geoph. 

 Union, 29, pp. 163-174, 1948. 



Press, F., Ewing, M., and Tolstoy, I., The Airy Phase 

 of Shallow-Focus Submarine Earthquakes, Bull. 

 Geol. Soc. Amer., 40, pp. 111-148, 1950. 



Press, F., and Ewing, M., Propagation of Explosive 

 Sound in a Liquid Layer Overlying a Semi-Infinite 

 Elastic Solid, Geophysics, 15, pp. 426-446, 1950. 



Ewing, M., Two Slow Surface Waves Across North 

 America, Bull. Seis7n* Soc. Amer., 43, pp. 219-228, 

 1952. 



Scholte, J. G., Over Het Verband Tussen Zeegolven 

 en Microseismen, I and II, Verslag Ned. Akad. 

 Wet., 52, pp. 669-683, 1943. 



Stonely, R., The Effect of the Ocean on Rayleigh 

 Waves, Mon. Not. Roy. Astron. Soc, Geophys. 

 Suppl., 1, pp. 349-356 1926. 



Whipple, F. J. W., and Lee, A. W., Note on the Theory 

 of Microseisms, Mon. Not. Roy. Astron. Soc, 

 Geophs. Suppl., 3, pp. 287-297, 1935. 



Discussion from the Floor 



(Dinger asked if the absence of periods 

 of one to seven seconds across the western At- 

 lantic was also true of the Pacific, and Press, 

 replied yes, on the Aleutians to Hawaiian path.) 

 Bath. Dr. Scholte mentioned that the ratios 

 of horizontal and vertical amplitude of micro- 

 seisms at De Bilt were larger than could be 

 explained by his theory. The reason is obvi- 

 ously the very loose ground at De Bilt with 

 around 9 km. of sediments (in accord with the 

 theory of A. W. Lee). 



(Haskell pointed out that where there is a 

 big contrast in velocity between the surface 

 layer and the underlying medium, there may 



